Looking for the local experts (hopefully David) to chime in on recommendations for a 12" sub to marry up with my Syn 2 amp in a ported enclosure. Actually looking for advice on a high quality constructed box to go along with it that is no more than 15" wide so that I can fit it in the walkway of my Sanger V215 with the port and sub on the same side. Looking hard at the JL 12W6 v3...but are there better options with this amp? SQ and SPL are both important but I prefer accuracy to rattle.

Looking for the local experts (hopefully David) to chime in on recommendations for a 12" sub to marry up with my Syn 2 amp in a ported enclosure. Actually looking for advice on a high quality constructed box to go along with it that is no more than 15" wide so that I can fit it in the walkway of my Sanger V215 with the port and sub on the same side. Looking hard at the JL 12W6 v3...but are there better options with this amp? SQ and SPL are both important but I prefer accuracy to rattle.

The W3 in a single 4-ohm voice coil is your only choice in the JL Audio woofer line with a bridged Wetsounds Syn2 amplifier. The extra power over the thermal rating doesn't concern me.
The W6 is a dual 4-ohm voice coil only and that will not match up well with a 2-channel amplifier whether in stereo or bridged mode. If the Syn2 was a short term solution then you could pair it with a W6.
The largest difference between the W3 and W6 is excursion. And excursion requires a lot more power to drive and control, not to mention that higher excursion woofers will usually have a slightly lower internal efficiency. So the W6 only offers an advantage when powered properly.
A subsonic filter is a nice feature anytime you are running a bass-reflex sub. Not essential, just of benefit.

Thanks David. I already have a Kicker 10 L5 ported connected to the Syn 2 and was hoping for something with more surface area and excursion, plus better sound. Not sure the 12 W3 will be much of an upgrade on any of those points.

how much space do you have? You could run 2 10w3 2 ohm in ported enclosure. That would be louder and sound better than what you have now.

With the Sanger bobsled seat base I am limited to almost truck box type dimensions if I go up under there. 22 wide x 7 bottom depth 10 top depth x 15 high. My Kicker TS10L5 just barely fits up under there. I have no interest in cutting the seat base up or removing it.

The other option is since I have the playpen seat cushion I could just put a box in the walkway up against the bow playpen cushion and that space only requires it be less than 15" wide to fit in the walkway and 24" or so long.

Also looking at the Wet Sounds XS-12 and that may be a better fit for the Syn 2 that I already have to run it. David, I see Earmark has some prebuilt water resistant boxes any idea which one would marry to the XS-12?

That depends on the particular application and the specific amplifier.
An open boat requires more power than an enclosed vehicle cabin.
A strictly regulated amplifier will get it done with less power than an unregulated amplifier.
A 10" W6 doesn't need quite the power as a 12" W6 even though the thermal capacity might be similar.
In a boat, and given the conservative power ratings by JL, I like to go to the very top of the thermal rating, and often more with an unregulated amplifier.
CEA-2006 ratings are way too easy to be a realistic measurement of actual performance. 14.4 volts which you won't have. At 12.5 volts you might drop 25% or more power. 1 kHz which is just an upper midrange frequency. You could easily lose another 20% at 20 Hz. On multi-channel amplifiers you may see another major drop with all channels driven versus one channel driven. At 40 Hz with all channels under load with a 12.0 V supply you could easily be half of the rated power.
Usually with experience a shop/dealer settles into combinations that they know to work well together. And that becomes their 'go to' package. Sometimes you try something and your expectations aren't met based on previous experiences with a given woofer. You make the change and your pleased.
So again, start with the thermal rating as a minimum threshold. Know that you can exceed that by some margin with good results and long term reliability if the system execution is right (design, tuning, etc.) and you know how to take care of your possessions.

Also looking at the Wet Sounds XS-12 and that may be a better fit for the Syn 2 that I already have to run it. David, I see Earmark has some prebuilt water resistant boxes any idea which one would marry to the XS-12?

If you are going to maintain the Syn2 amplifier then I would go the XS-12 route before the W6 route. The XS-12 is the rough equivalent of the 12W3 and either woofer will be more surface area than a 10" sub, whether round or square. By far, surface area is the most efficient way to get more output.
As for the enclosures, you'll need to get with Odin at Earmark Marine for that information. Odin definitely knows audio so he is a great resource.

David, could I run something like the CS210G-TW3: Dual 10TW3 ProWedge, Sealed, Dual 8 Ω Terminals with the Syn 2 and what's your opinion on sound quality on these JL premade boxes?

That is a very impressive sounding subwoofer. However, you don't want to use that type of cabinet construction in a boat. Also, you do not want to orient those woofers front and rear. Side-firing would require about 5 to 6-inches of open clearance on each side and I can't imagine how that would work.

That is a very impressive sounding subwoofer. However, you don't want to use that type of cabinet construction in a boat. Also, you do not want to orient those woofers front and rear. Side-firing would require about 5 to 6-inches of open clearance on each side and I can't imagine how that would work.

Are you recommending side firing as the choice in the under dash location as well for any subwoofer, or just that this particular JL sub box requires side firing to be optimal?